A polymer electrolyte fuel cell is a system for obtaining electric power using pure hydrogen, hydrogen gas obtained by conversion of alcohols, etc. as a fuel and electrochemically controlling the reaction with hydrogen and oxygen in the air.
A polymer electrolyte fuel cell uses a hydrogen ion selective permeation type membrane as an electrolyte, can be made more compact compared with a conventional alkali type fuel cell, phosphoric acid type fuel cell, molten carbonate type fuel cell, solid electrolyte type fuel cell, or other such fuel cell using as an electrolyte an aqueous solution-type electrolyte, molten salt type electrolyte, or other fluid medium, and is being worked on for application to electric vehicles etc.
The configuration of a typical solid polymer type fuel cell is shown in FIG. 1. That is, the polymer electrolyte fuel cell 1 is comprised of a hydrogen ion selective permeation type membrane 2 forming an electrolyte, a catalyst electrode part 3 comprising carbon fine particles and precious metal superfine particles provided on the two sides of this membrane 2, a current collector having the functions of taking out electrode power generated at this catalyst electrode part 3 as current and supplying reaction gas to the catalyst electrode part 3, that is, oxygen-based gas or hydrogen-based gas (usually carbon paper 4), and a separator 5 receiving current from the carbon paper 4 and separating the oxygen-based gas and hydrogen-based gas.
The basic principle of a polymer electrolyte fuel cell 1 is generally as follows. That is, in a polymer electrolyte fuel cell 1, the fuel, that is, the hydrogen gas (H2) 8, is supplied from the anode side, passes through the gas diffusion layers of the carbon paper 4 and catalyst electrode part 3 to form hydrogen ions (H+) which permeate through the electrolyte, that is, the membrane 2. At the cathode side catalyst electrode part 3, an oxidation reaction (2H++2e−+½O2→H2O) occurs between the hydrogen ions (H+) and the oxygen (O2) in the air 9 supplied from the cathode side whereby water (H2O) is produced. At the time of this oxidation reaction, the electrons 10 produced at the anode side catalyst electrode part 3 flow through the carbon paper 4 from the anode side separator 6 to the cathode side separator 7 whereby current and voltage are produced across the electrodes.
The membrane 2 comprises an electrolyte having a strong acidity fixed in a film and controls the dew point in the cell to function as an electrolyte for permeation of hydrogen ions (H+).
A separator 5, a component member of a polymer electrolyte fuel cell 1, separates the two types of reaction gas, that is, the cathode side air 9 and anode side hydrogen gas 8, and performs the role as flow paths for the supply of the reaction gases and the role of exhaust the water produced by the reaction from the cathode side. Further, in general, the polymer electrolyte fuel cell 1 uses a membrane comprised of an electrolyte exhibiting a strong acidity. Due to the reaction, it operates at a temperature of about 150° C. or less. Water is produced, so the separator 5 of the polymer electrolyte fuel cell is required to have the material properties of corrosion resistance and durability and is required to have a good conductivity for efficiently conducting the current through the carbon paper 4 and a low contact resistance with the carbon paper.
In the past, carbon-based materials had been frequently used as the material for separators of polymer electrolyte fuel cells. However, separators comprised of carbon-based materials cannot be made thinner due to the problems of brittleness, so have obstructed compactness. In recent years, separators made from hard to break carbon-based materials have also been developed, but these are high in cost and therefore disadvantageous economically.
On the other hand, separators using metal materials do not have the problem of brittleness compared with carbon-based materials, so in particular greater compactness and further lower cost of the polymer electrolyte fuel cell system become possible. Therefore, separators using titanium or other metal materials are being developed and numerously proposed (for example, see Japanese Patent Publication (A) No. 2000-260439 and Japanese Patent Publication (A) No. 2000-256808).
However, there was the problem that separators made of titanium and titanium alloys became larger in contact resistance with the carbon paper due to the passive film formed on the surfaces and the energy efficiency of the fuel cells was greatly reduced.
For this reason, in the past, numerous methods have been proposed for reducing the contact resistance between a titanium separator surface and carbon paper.
For example, a separator for a polymer electrolyte fuel cell has been proposed which makes a noble metal or noble metal alloy bond with a titanium or stainless steel surface to reduce the contact resistance with carbon paper (for example, see Japanese Patent Publication (A) No. 2001-6713) etc. However, these methods had problems in that surface treatment forming a gold plating or other expensive noble metal layer for imparting conductivity on a titanium or stainless steel surface is required, so the cost of production of the separators increased.
On the other hand, various methods are also being proposed for reducing the contact resistance between the surface of separator materials and carbon paper while reducing the amount of use of expensive noble metal or not using them at all.
Further, the method of affixing hard fine powder having conductivity to the substrate surface by shot etc. has also been proposed. For example, a separator made of titanium in which M23C6 type, M4C type, or MC type conductive hard particles, where the metal element (M) includes one or more of chromium, iron, nickel, molybdenum, tungsten, and boron, are buried and dispersed in and exposed at the substrate surface (for example, see Japanese Patent Publication (A) No. 2001-357862) has been proposed.
The method of affixing such hard fine powder having conductivity to the substrate surface by shot etc. does not reduce the productivity compared with the method of using heat treatment or vacuum deposition and is advantageous in the point of being inexpensive in production costs and simple in method. On the other hand, in the method of mechanically driving hard conductive particles into a surface of a substrate of a metal separator molded to a desired shape by the blast method etc., strain is introduced into the substrate surface layer part causing the possibility of deformation and the separator sometimes falls in flatness.
In general, a solid polymer type fuel cell has an output voltage per cell of a low 1V or so, therefore to obtain the desired output, a large number of fuel cells are often stacked for use as stacked fuel cells. For this reason, in the method of affixing hard fine powder having conductivity to the substrate surface by shot etc., it is necessary to set the conditions and perform post-treatment to obtain a separator suppressing warping and strain in the separator and having a good flatness enabling stacking of the fuel cells.
In the above way, in the past, a metal separator of a polymer electrolyte fuel cell using as the separator substrate a titanium or other metal material superior in corrosion resistance and improving the contact resistance between the surface of the separator substrate and carbon paper by forming a conductive compound layer on the substrate surface by various metals or affixing conductive compound particles has been proposed, but this could not be said to have been sufficient from the viewpoint of the contact resistance and flatness required as a polymer electrolyte fuel cell separator and further the moldability or from the viewpoint of the productivity and production costs.
Further, from the inventors' study of the prior art, they learned that in a titanium or other metal separator affixing a conductive compound to the substrate surface to reduce the contact resistance of the separator surface, at the time of use of the fuel cell, there is the problem that metal ions are eluted from the conductive compound of the substrate surface into the MEA (membrane electrode assembly), the electromotive force falls and other battery characteristics deteriorate, and the power generation capability falls. On the other hand, in a metal separator comprised of a substrate on the surface of which a noble metal is plated or embedded as a conductive substance, there is no such problem, but as explained above, use of a noble metal has the problems of limited resources and higher production costs.